OPTOv4: Dual-channel passive optocoupler VCA/filter/low-pass gate (4HP)

BE AWARE! This is a deliberately lo-fi module - if you want crystal clear sound reproduction, fidelity or whatever then you won't find it here.

AN UPDATED VERSION OF THIS MODULE IS AVAILABLE - the updated design looks outwardly identical to its predecessor but there are two important changes to the circuit. Firstly, the two channels are no longer normalled together by default - if you wish to restore this, bridge the solder jumper on the back of the board. Secondly, a capacitor has been added to each channel to reduce the amount of high-frequency bleedthrough (thanks to Andy Butler for the idea)

The module relies on the properties of a photo-transistor to generally dirty up any signal coming passing through it; think of it as kind-of like a low-pass gate with built in distortion and a few other properties too.

In the beginning there was the OptoThing - it was an interesting piece of kit in that it sometimes behaved a bit like a VCA, or maybe a filter, or maybe somewhere in between. It was a classic example (I think) of how something with a relatively low parts count could do weird and wacky things to your sound. This is an updated version - revised layout, mostly surface-mount construction (cheaper) and two Big-Ass Knobs[tm].

Operation is the same - give it a CV, and input and an output and twiddle the pot until you get a response that you like. The CV inputs aren't normalled together like they used to be - if you want that, put a blob of solder

Use it as a 'ghetto' VCA with an envelope-type waveform or use swept voltages to create bizarre, filter-like effects. Whilst the opportunities aren't quite endless, there are still quite a few of 'em.

Panels are made from 1.6mm FR4 (PCB material). There's no power cable as this is a fully passive module

SUPPLIED KNOB DESIGNS MAY VARY - THIS DOES NOT AFFECT FUNCTIONALITY

You can hear it in action here: https://soundcloud.com/yorkmodular/optothing-ghetto-vca🔗